1. Field of the Invention
This invention relates to a chemically amplified positive resist composition having blended therein a base resin in the form of a polymer having at least one acid labile group and crosslinked within a molecule and/or between molecules with a crosslinking group having a C--O--C linkage, the resist composition having a high alkali dissolution contrast, high sensitivity and high resolution and being improved in reproducibility as a fine patterning material for use in the manufacture of LSIs. It also relates to a pattern forming method and a method for preparing a polymer useful as the base resin of such a resist composition.
2. Prior Art
As the LSI technology tends toward higher integration and higher speed, further refinement of pattern rules is required. Deep-ultraviolet lithography is regarded promising as the next generation of fine patterning technology. The deep-UV lithography is capable of working on the order of less than 0.5 .mu.m and makes it possible to form a resist pattern having a side wall nearly perpendicular to the substrate if a less light absorbing resist material is used.
From this point of view, a number of chemically amplified positive working resist materials using acid catalysts were recently developed as disclosed in JP-B 27660/1990, JP-A 27829/1988, U.S. Pat. Nos. 4,491,628 and 5,310,619. These materials have high sensitivity, resolution and dry etching resistance and are promising as resist materials especially suited for deep-UV lithography.
Known chemically amplified positive working resist materials include a two-component system comprising a base resin and a photoacid generator and a three-component system comprising a base resin, a photoacid generator, and a dissolution controller having an acid labile group. For example, JP-A 115440/1987 discloses a resist composition comprising poly-4-tert-butoxystyrene and a photoacid generator. There are proposed analogous resist compositions. JP-A 223858/1991 discloses a two-component resist composition comprising a resin having a tert-butoxy group in a molecule and a photoacid generator. JP-A 211258/1992 discloses a two-component resist composition comprising a polyhydroxy-styrene having a methyl, isopropyl, tert-butyl, tetrahydro-pyranyl or trimethylsilyl group and a photoacid generator. Further, JP-A 100488/1994 discloses a resist composition comprising a polydihydroxystyrene derivative such as poly(3,4-bis(2-tetrahydropyranyloxy)styrene), poly(3,4-bis(tert-butoxycarbonyloxy)styrene), and poly(3,5-bis(2-tetrahydropyranyloxy)styrene) and a photoacid generator.
However, the base resins used in these resist compositions have an acid labile group in their side chain. If acid labile groups such as tert-butyl and tert-butoxycarbonyl groups are decomposable only with strong acid, decomposition of acid labile groups is retarded because they are deactivated through reaction with air-borne basic compounds. As a result, resist material tends to form a pattern of T-top profile. On the other hand, alkoxyalkyl groups such as ethoxyethyl are decomposable with weak acid. Base resins having such alkoxyalkyl groups are little influenced by air-borne basic compounds, but suffer from problems including significant thinning of the pattern shape with the lapse of time from exposure to heat treatment, low heat resistance due to the presence of bulky groups in a side chain, and unsatisfactory sensitivity and resolution. None of these resins have been fully acceptable in practice. There is a desire to overcome these problems.
It is noted that the polymer described in JP-A 305025/1996 aims to overcome the above-mentioned problems. Due to some restrictions on the preparation method, it is difficult to design a desired percent substitution of an acid labile group and a crosslinking group. The preparation method often results in formation of undesirable crosslinking groups as described in JP-A 253534/1996. More particularly, on the design of a resist composition, a variety of polymers having a varying alkali dissolution rate are required so as to comply with the type and amount of a particular photoacid generator and additive. It is also required to prepare such a polymer in a reproducible manner. The preparation method described in the above-cited patent reference is restricted with respect to the selection of an acid labile group and crosslinking group and the percent substitution thereof.
Under the circumstances, attention is now paid to crosslinked polymers as a base resin.
In the prior art, as typified by negative resist materials, crosslinking reaction is generally carried out by crosslinking a resin comprising hexamethoxymethylmelamine and polyvinyl phenol with an acid for insolubilizing the resin. That is, crosslinking reaction aims to insolubilize the resin. Where a resin is used as a positive resist material, however, the resin must be soluble in a solvent. Since conventional crosslinking reaction is to insolubilize resins, the crosslinked products cannot be used as a positive resist material.
For use as a positive resist material, exposed areas must become soluble. However, conventional crosslinking reaction with hexamethoxymethylmelamine, etc. cannot produce a resin which is decomposed to be alkali soluble.
Yamaoka et al. report in Polymers for Advanced Technologies, vol. 5, 499-506; and Chem. Mater., vol. 6, No. 10 (1994), 1854, that by heat crosslinking of a mixed system of a polyvinyl phenol derivative and a divinyl ether compound PAG, there are obtained products which are decomposable by exposure and thus suitable for use as a positive resist material. The heat crosslinking reaction between a polyvinyl phenol derivative and a divinyl ether compound, however, has several problems of a long reaction time, difficult control of a crosslinking rate, and unreacted divinyl ether compound as a residue.
As to crosslinking reaction of an acetal group, it is known to carry out reaction in the presence of an acid, vinyl ether compound, and diol. In this method, both crosslinking reaction and acetal-forming reaction of a side chain occur. It is impossible to select either one of the reactions. Because of the competitive reaction between crosslinking reaction and acetal-forming reaction of a side chain, it is very difficult to control a rate of reaction. It is also a problem that a crosslinked reaction product which is least decomposable with acid forms in this reaction process.